Or remove incentives for having multiple accounts in the first place (even multiple public identities should be something handled by your system without need fo

Or remove incentives for having multiple accounts in the first place (even multiple public identities should be something handled by your system without need for re-log), and stop messing with emails. On one hand, even your assumption that a shared inbox implies one person is wrong (I don't like it when people share inboxes and consider it toxic, but it is what it is), and your mitigations are futile (it's between cheap and free to just have an entire domain point to your one inbox - your 'federated gmail' is easily such a case).